1. Field of the Invention
This invention relates generally to a TV lens, and more particularly to a TV lens that simplifies the adjustment of a flange back length by utilizing the auto-focusing function.
2. DESCRIPTION OF RELATED ART
When a TV lens is attached to a TV camera, it is necessary to adjust a flange back length, which is the distance between an attachment plane (a flange plane) of the lens and a focal plane of the lens, so that the focal plane of the lens can match with an imaging surface of an imaging device (e.g., a CCD) in the camera.
To adjust the flange back length, a) an iris of the lens is opened, b) a zoom lens is set at a telephoto end and a focus lens is moved so that the TV lens can be focused, and c) the zoom lens is set at a wide end and an adjusting control, which is provided close to a lens mount, is turned to move a master lens so that the TV lens can be focused. Then, the above-mentioned steps b) to c) are repeated several times.
Adjusting the flange back length in the above-mentioned manner prevents the shift of the focal plane of the TV lens between cases where the zoom lens is placed at a telephoto side and the zoom lens is placed at a wide side.
The conventional TV lens, however, has a disadvantage because a cameraman has to adjust the flange back length manually, and thus, the adjustment is complicated.
The present invention has been developed in view of the above-described circumstances, and has as its object the provision of a TV lens that enables the automatic adjustment of the flange back length, thereby relieving the work load of the cameraman.
To achieve the above-mentioned object, the present invention is directed to a TV lens attached to a TV camera, the TV lens comprising: a focus lens group; a zoom lens group; a master lens group; a focus lens drive means for moving the focus lens group with a first motor; a zoom lens drive means for moving the zoom lens group with a second motor; a master lens drive means for moving the master lens group with a third motor; a first focusing means for focusing the TV lens by controlling the focus lens drive means according to a focus evaluation value representing a sharpness of an image, the image being projected on an imaging plane of the TV camera by the TV lens, the focus evaluation value being produced from a video signal converted from the image; a second focusing means for focusing the TV lens by controlling the master lens drive means according to the focus evaluation value; and a control means for adjusting a flange back length of the TV lens by controlling the zoom lens drive means and the first and the second focusing means such that focusing of the TV lens by the first focusing means while the zoom lens group is placed at a telephoto side and focusing of the TV lens by the second focusing means while the zoom lens group is placed at a wide side are repeated alternately at least once.
According to the present invention, when the TV lens is initialized, the zoom lens drive means places the zoom lens group at the telephoto side, and the first focusing means moves the focus lens group to focus the TV lens according to the focus evaluation value, thereafter the zoom lens drive means places the zoom lens group at the wide side, and the second focusing means moves the master lens group to focus the TV lens according to the focus evaluation value.
The procedure may be as follows: the zoom lens drive means places the zoom lens group at the wide side first, and the second focusing means moves the master lens group to focus the TV lens according to the focus evaluation value, thereafter the zoom lens drive means places the zoom lens group at the telephoto side, and the first focusing means moves the focus lens group to focus the TV lens according to the focus evaluation value.
As stated above, the focusing with the focus lens group while the zoom lens group is placed at the telephoto side and the focusing with the master lens group while the zoom lens group is placed at the wide side are repeated alternately several times, so that the focal plane of the TV lens can match with the imaging surface of the imaging device in the TV camera. Thus, the flange back length of the TV lens can be adjusted automatically, and the cameraman is saved from a great deal of trouble.
A manipulatable commanding means may be provided that commands the control means to start to adjust the flange back length of the TV lens. When the cameraman operates the commanding means, the initialization (the automatic adjustments of the flange back length) of the TV lens is started.
According to another embodiment of the present invention, the TV lens further comprises: a memory means for memorizing respective positions of the focus lens group and the zoom lens group when the commanding means commands the control means to start to adjust the flange back length of the TV lens; and a second control means for controlling the focus lens drive means and the zoom lens drive means such that the focus lens group and the zoom lens group are returned to their respective memorized positions after the flange back length of the TV lens is adjusted.
Consequently, after the flange back length is adjusted in accordance with the command from the commanding means, the focus lens group and the zoom lens group can automatically be returned to their respective positions before the initialization, so that the shooting can be performed continuously.
Furthermore, while the flange back length is adjusted, a display means indicates that the flange back length is currently adjusted. This enables the cameraman to easily know whether the initialization is in operation.